Seed storage proteins are conventionally classified into four groups of proteins, according to their solubility, i.e., glutelin, globulin, prolamin, and albumin. Rice is different from other grains, such as wheat and maize, in that glutelin is the major seed storage protein, accounting for about 70 to 80% of the seed storage proteins. The glutelin gene group comprises about 10 genes per one haploid genome, and the genes are divided into two subfamilies, GluA and GluB, which show a homology of 60 to 65% at the amino acid sequence level within the coding region. Each subfamily comprises about 5 genes that have a homology of 80% or higher at the amino acid sequence level. A glutelin gene is specifically expressed and accumulated in the endosperm. The tissue specificity of glutelin expression is considerably strictly regulated, and glutelins are not expressed in other tissues, such as leaf and root. The expression of the glutelin gene group, with the exception of GluA-3, is generally coordinated; their mRNA levels show the following pattern: emerging 5 days after flowering (day 5), reaching maximum at around day 15, and decreasing thereafter. The GluB-1 gene has the strongest promoter activity in the glutelin gene group.
Rice mutants with decreased amount of accumulated glutelin, i.e., the major seed storage protein, have been isolated. For example, Iida et al. isolated recessive mutants that lack either one of the acidic subunits of glutelin, α1, α2, or α3, from a rice breed Koshihikari that was irradiated with γ-ray. The phenotypes are respectively regulated by a single recessive gene (i.e., glu1, glu2, or glu3). A mutant strain (α123) that lacks all of α1, α2, and α3 has been also obtained by crossing the above three mutants (Iida, S. et al., Theor. Appl. Genet. 94: 177-183 (1997)).
LGC-1 (low glutelin content-1) is a mutant selected from Nihonmasari treated with EMS, and has a phenotype with a significantly reduced level of glutelin (Iida, S. et al., Theor. Appl. Genet. 87: 374-378 (1993) ). LGC-1 is further characterized by increased levels of prolamin and globulin. LGC-1 is dominated by a single dominant gene. By mapping the defective genes in LGC-1 and the mutants defective of α1, α2, and α3, it was revealed that the mutated protein gene (lgc-1) in LGC-1 and the mutated glutelin gene (glu1) in the mutant lacking α1 are localized on the same locus. The results of Southern hybridization using the glutelin (GluB) gene as a probe suggested that LGC-1 contained a mutation in the GluB gene or in the proximity thereof. According to the results of Northern blot analysis, comparing the expression level of the GluB gene in the endosperm after about 16 days from head spout in LGC-1 and its original breed, Nihonmasari, it was revealed that GluB expression is markedly decreased in LGC-1.
In soybean, glycinin is known as a seed storage protein. Glycinin is produced as a precursor polypeptide of a size of about 60 kDa wherein a signal peptide, an acidic polypeptide, and a basic polypeptide are bound together; the signal peptide is cleaved afterwards. Thereafter, a subunit is formed wherein two kinds of polypeptides that result from a cleavage at the Asn-Gly site—i.e., the specific acidic polypeptide (A) and basic polypeptide (B)—are polymerized through disulfide bonds. Six of these subunits are assembled to form a hexamer, and are stored in the protein body (PB) The hexamer is also called “11S seed storage protein”, due to its sedimentation coefficient (11S). Glycinin subunits are classified into group I and group II based on the primary structure of their cDNAs and their amino acid sequence homology. To date, subunits A1aB1b, A1bB2, and A2B1a of group I, and subunits A3B4, and A5A4B3 of group II are known. Six of these subunits are known to be almost randomly combined in soybean glycinin. Furthermore, a peptide derived from the A1aB1b subunit of soybean glycinin has been reported to have the ability to bind to bile acid (Shio Makino, The Food Industry 39(24): 77-87 (1996)), which suggests that the ability of soybean proteins to decrease the cholesterol level in blood is dependent on the A1aB1b subunit.